1. Related Field
The present invention relates to digital image processing and, more specifically, for reducing block artifacts in a block-encoded image.
2. Background
Block-based discrete cosine transform (DCT) is a compression technique used for many standard video compression methods including ITU-T H261, H263, H264, ISO JPEG, MPEG-1, and MPEG-2. To perform a DCT, an image is first divided into blocks. DCT is applied to each row and column of each of the blocks to produce a transform coefficient array. In a typical transform coefficient array the (0,0) element in each block is the DC (zero-frequency) component, and entries with increasing vertical and horizontal index values represent higher vertical and horizontal spatial frequencies of the image.
One common problem with block-based DCT is that it can produce perceptible artifacts when an image is compressed at low bit rates. One such artifact is the appearance of block structures that are particularly visible in smooth-toned regions of the image. In an image sequence, an insufficient bit rate allocation in the compression encoder can also exacerbate blocking artifacts in fast moving regions or during scene changes. While there are existing techniques for block artifact reduction (BAR), most existing methods require known or easily detectable block borders and/or block sizes. However, some common image manipulations such as cropping and multi-generation re-encoding can make block border locations difficult to detect. When images are not produced using proper image manipulation techniques, which is common in media such as online video streaming, the block border information is often distorted or lost. As such, many known methods for BAR cannot be used.
Therefore, there is a need to provide efficient block artifact reduction methods more suitable for image sequences with unknown block sizes and also able to reduces blocking artifacts in fast moving regions.